1. Field of the Invention
The present invention relates to a friction engaging device which is used for applying or releasing a torque supplied from a driving force source.
2. Related Art
Disclosed in JP-A-2003-113874 is a friction engaging device applied to an electric motor driving device. The electric motor driving device decelerates an output of an electric motor and transmits the decelerated output to right and left axle shafts for driving rear right and left wheels. An electric motor is used as a sub-driving source. Front wheels use an engine such as an internal combustion engine as a main driving source, thus front right and left wheels are driven by the engine.
In the electric motor driving device, a first power transmission shaft is rotatably supported to a fixed housing. The first power transmission shaft is interlockingly connected with the electric motor so as to receive the input torque from the electric motor. A first reduction gear constituting a first reduction mechanism is provided in the first power transmission shaft. A second reduction gear of the first reduction mechanism located on the other side is engaged with the first reduction gear. The second reduction gear is supported to a second power transmission shaft. The second power transmission shaft is arranged in parallel to the first power transmission shaft and rotatably supported to the housing.
A third reduction gear constituting a second reduction mechanism is provided in the second power transmission shaft. A fourth reduction gear of the second reduction mechanism located on the other side is engaged with the second reduction gear. The fourth reduction gear is supported to relatively rotate about a differential case of a rear differential device by a bearing.
The rear differential device supports a differential gear mechanism in the differential case. The differential case is rotatably supported to the housing by a bearing.
Switching to apply and release the torque between the fourth reduction gear and the differential case is performed in a main clutch constituted by using a multi friction plate of an electromagnetic friction clutch.
If the electromagnetic friction clutch is in a torque transmission state, the reduced torque is transmitted to the rear differential device via the first and second reduction mechanisms by driving the electric motor. The torque is transmitted to the right and left axle shafts from the rear differential device. The torque assists an engine driving in a launch driving or a climb driving.
However, an outer plate of the electromagnetic friction clutch is formed in, for example, a friction plate made of a SC material (carbon steel) and a surface of an inner plate thereof is formed in a friction plate having a friction material such as a paper or a carbon, or the like. Further, generally, a friction engaging device of a driving force transmission system such as the electromagnetic friction clutch is under a lubricated environment by a lubricant including a friction adjuster (FM material, a friction modifier).
Therefore, because the FM material included in a lubricant is chemically adsorbed into the surface of the outer plate made of the SC material and the surface thereof is activated, depending on a temperature environment, there is a problem in which a friction coefficient thereof is declined.